Compact Cross-Dispersed Spectrometer for Extended Spectral Range

ABSTRACT

A spectrometer includes a structural member made of a light-weight material having a small coefficient of thermal expansion (CTE). The spectrometer is dimensionally stable over a range of expected ambient temperatures, without controlling the temperature of the spectrometer.

The present application claims priority from U.S. Provisional PatentApplication No. 60/891,408, titled “Hand-Held, Self-Contained OpticalEmission Spectroscopy (OES) Analyzer,” filed Feb. 23, 2007, which isincorporated in its entirety by reference herein.

The contents of commonly-assigned U.S. patent application Ser. No.12/035,477, by Denis Baiko, et al., titled “Fast and PreciseTime-Resolved Spectroscopy with Linear Sensor Array,” filed Feb. 22,2008, is incorporated in its entirety by reference herein.

The contents of commonly-assigned U.S. patent application Ser. No.______ (to be supplied), by John E. Goulter, et al., titled “Hand-Held,Self-Contained Optical Emission Spectroscopy (OES) Analyzer,” filed Feb.22, 2008, is incorporated in its entirety by reference herein.

TECHNICAL FIELD

The present invention relates to spectrometers and, more particularly,to compact spectrometers that remain dimensionally stable and accurateover a temperature range without heating or cooling.

BACKGROUND ART

Analyzing chemical composition of samples is important in many contexts,including identifying and segregating metal types (particularly varioustypes of iron and steel) in outdoor metal recycling facilities, qualitycontrol testing in factories and forensic work. Several analyticalmethods are available.

Optical emission spectroscopy (OES) is a mature, robust technology forthe elemental analysis of materials. In OES, a small quantity of samplematerial is vaporized and excited above atomic ground state. Emissionscharacteristic of elements in the vaporized sample are captured by alight guide, which sends the light to a spectrometer, which produces andanalyzes a spectrum from the light, so as to yield the elementalcomposition.

For metal samples, the prevalent techniques for generating an emissionspectra use either an electric arc or a spark, or both, to vaporize asmall quantity of the sample to be analyzed. Alternatively,laser-induced breakdown spectroscopy (LIBS) or glow discharge (GD) maybe used to vaporize and excite an emission sample. A survey of OESanalytical techniques may be found in Slickers, AutomaticAtomic-Emission Spectroscopy, Second Edition (1993), which isincorporated by reference as if fully set forth herein.

In order to be confident that the composition deduced from ameasurement, which typically tests a miniscule portion of the sample, isrepresentative of the composition of the entire sample, minimizingeffects from, for example, inclusions, matrixes and surfacecontaminants, it is standard practice to average the spectra from asmany as several thousand arcs/sparks that have struck an area as largeas 100 square mm in a few seconds of a measurement.

Some OES analyzers are large, non-portable units intended for use inlaboratories. Other OES analyzers are “portable,” in that they can bemoved about. However, prior art “portable” OES analyzers that canidentify carbon or other common constituents in iron or steel requiretwo separate components interconnected by a fiber optic/electric cable.For example, an analyzer available from Spectro A. I., Inc. under thetrade name Spectroport includes a hand-held probe connected via a10-foot cable to a suitcase-sized, 33-pound analysis unit. The SpectroiSort analyzer, also from Spectro A. I., Inc., includes a hand-heldprobe connected by a cable to an analysis unit housed in a 10-poundbackpack.

To cover a spectral range required to detect carbon, phosphorous, sulfurand other elements necessary to identify common materials, such as castiron and various alloys, these prior art analyzers includefixed-wavelength detectors in the hand-held probes for carbon,phosphorous, sulfur and iron, as well as a spectrometer in the analysisunit for other elements. This awkward, two-part structure makes theseanalyzers difficult to use and move about.

An two-part analyzer available from Metorex, Ewing, N.J., under thetrade name ARC-MET 8000 MobileLab, includes a hand-held “probe”connected by a ten-foot cable to a roll-around “main unit.” The probecontains a spectrometer with an advertised spectral range of 175-370 nm;however, the roll-around main unit is required to provide power andcooling to the probe and to analyzes the output from the spectrometer.At least some users would prefer to use a hand-held OES analyzer that isfully self-contained.

The Spectrosort analyzer, also from Spectro A. I., Inc., is a one-piece,battery-powered, hand-held analyzer. However, spectral limitations ofthe spectrometer in this analyzer make it incapable of detecting carbon,phosphorous and sulfur, thus severely limiting the utility of thisanalyzer.

Users of self-contained, hand-held OES analyzers would prefer analyzersthat are capable of detecting carbon and other key elements, so theanalyzers can identify a wide range of common materials. However,various roadblocks have thus far prevented construction of such afull-range, self-contained, hand-held analyzer. Among these roadblocksis an inability to construct a spectrometer that exhibits the wavelengthrange and temperature stability needed for the above-described analysisunder typical environmental conditions, in a size and weight appropriatefor a hand-held analyzer,

SUMMARY OF THE INVENTION

An embodiment of the present invention provides an analyzer foranalyzing composition of a portion of a sample. The analyzer includes ahand-held, self-contained, test instrument. The test instrument includesan exciter for exciting the portion of the sample, the excitationproducing an optical signal and a first dispersive element disposedwithin the hand-held instrument for receiving the optical signal andcreating an intermediate optical signal dispersed in a first plane. Asecond dispersive element disposed within the hand-held instrumentdisperses the intermediate optical signal so as to place a firstresolved optical order on a corresponding first plurality of detectorelements and a second resolved optical order on a corresponding secondplurality of detector elements. A processor is coupled to receivesignals from the first and second pluralities of detector elements andis programmed to process the signals. A battery powers the exciter andthe processor.

At least one of the optical orders placed on the corresponding pluralityof detector elements may extend to wavelengths shorter than about 193nm, or shorter than about 178 nm, or at least as short as about 170 nm.

Each plurality of detector elements may be configured so as to receive acontinuous spectral range of the resolved optical order placed on theplurality of detector elements. The spectral range placed on the firstand second pluralities of detector elements may extend at least fromabout 178 nm to about 400 nm.

The instrument may include a structure defining an aperture, throughwhich the intermediate optical signal passes. The optical signal may befocused on the structure.

The exciter may include an electrode for sustaining an electricalpotential with respect to the portion of the sample and a voltage supplyfor establishing the electrical potential on the electrode with respectto the portion of the sample. The exciter may include a laser.

The first dispersive element may be a cross-dispersing prism. The seconddispersive element may be a diffraction grating, which may be aholographic diffraction grating blazed to provide comparableefficiencies in the first and second resolved optical orders.

The first plurality of detector elements may be not co-planar with thesecond plurality of detector elements. The test instrument may furtherinclude a mirror in an optical path of one of the first and secondresolved optical orders, between the second dispersive element and thecorresponding plurality of detector elements.

The first and second dispersive elements and the first and secondpluralities of detector elements may be rigidly coupled to acarbon-filled polymer structural member, which may include polyphylenesulfide filled with graphite, such as with at least about 40% graphite.

The processor may be programmed for automatic wavelength calibration,based on observed spectral features.

The second dispersive element may provide a resolving power of at leastabout 5,000 or at least about 10,000.

The instrument may further include a display screen coupled to theprocessor. The display screen may be a hinged display screen.

An embodiment of the present invention provides an analyzer foranalyzing composition of a portion of a sample. The analyzer may includea hand-held, self-contained, test instrument that includes an exciterfor exciting the portion of the sample. The excitation produces anoptical signal. The instrument also includes a spectrometer having aspectral range extending at least from about 178 nm to about 400 nmdisposed in the analyzer to receive the optical signal and operative todisperse the optical signal and produce an output signal from thedispersed optical signal. The instrument also includes a processorcoupled to the spectrometer and programmed to process the output signaland a battery powering the exciter, the spectrometer and the processor.

The spectrometer may include a pixilated sensor, and the spectrometermay have a resolution of at least about 0.02 nm per pixel at about 190nm.

The spectrometer may include a holographic diffraction grating havingcomparable efficiency in at least two different orders and sensorsarranged to receive two orders of the dispersed optical signal from thegrating. The spectrometer may be cross-dispersed.

The spectrometer may include a structural member that includes acarbon-filled polymer, to which optical elements of the spectrometer aremounted.

The processor may be programmed to automatically wavelength calibratethe spectrometer, based on observed spectral features.

Another embodiment of the present invention provides a method foranalyzing composition of a portion of a sample. The method includesexciting the portion of the sample, thereby producing an optical signaland generating a spectrum from the optical signal. A first predeterminedspectral feature is matched with at least a portion of the spectrum. Awavelength is associated with a pixel, based on a location of the firstpredetermined spectral feature, relative to the pixel. The spectrum isanalyzed to determine at least one constituent of the portion of thesample.

Wavelengths may be associated with other pixels, based on an expectedlinear spectral dispersion over a set of pixels.

A second predetermined spectral feature may be matched with at least aportion of the spectrum and wavelengths may be associated with otherpixels, based on a location of the second predetermined spectralfeature, relative to the location of the first predetermined spectralfeature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by referring to thefollowing Detailed Description of Specific Embodiments in conjunctionwith the Drawings, of which:

FIG. 1 is a perspective view of a hand-held, self-contained,battery-powered OES test instrument, according to an embodiment of thepresent invention;

FIG. 2 is a cut-away view of the test instrument of FIG. 1;

FIG. 3 is a perspective view of a spectrometer of the instrument of FIG.1, according to an embodiment of the present invention;

FIG. 4 is a close-up perspective view of a portion of the spectrometerof FIG. 3;

FIG. 5 is shows a snout of the test instrument of FIG. 1 in contact witha sample surface;

FIG. 6 is a graph showing representative voltage and current curvesplotted against time for a single spark/arc generated by the instrumentof FIG. 1, according to one embodiment of the present invention;

FIG. 7 contains two graphs showing representative voltage and currentcurves plotted against time for sparks/arcs generated by the instrumentof FIG. 1, according to one embodiment of the present invention;

FIG. 8 is a close-up schematic view of an analytical gap produced by thetest instrument of FIG. 1;

FIG. 9 is a cutaway perspective diagram of the spectrometer of FIGS. 3and 4, according to one embodiment of the present invention;

FIG. 10 contains two schematic representations of spectra projected onsensor arrays of the instrument of FIG. 1, in relation to selectingsystem parameters according to one embodiment of the present invention;

FIG. 11 is a schematic diagram two rows of sensors, according to oneembodiment of the present invention;

FIG. 12 is a perspective view of the two rows of sensors of FIG. 11 andcorresponding mounting brackets therefore, according to one embodimentof the present invention;

FIG. 13 is an exploded view of a diffraction grating assembly, accordingto one embodiment of the present invention;

FIG. 14 is a perspective view of the diffraction grating assembly ofFIG. 13 mounted within a spectrometer housing, according to oneembodiment of the present invention;

FIG. 15 is a schematic diagram of an alignment setup for the testinstrument of FIG. 1, according to one embodiment of the presentinvention;

FIG. 16 is a flow chart describing a process for analyzing compositionof a sample, according to one embodiment of the present invention;

FIG. 17 is a block diagram of major components of the test instrument ofFIG. 1, according to one embodiment of the present invention;

FIG. 18 is a perspective view of a hand-held, self-contained testinstrument with a tilt-up screen, according to one embodiment of thepresent invention;

FIG. 19 is a schematic graph illustrating two spectral peaks andcorresponding signals produced by sensor pixels;

FIGS. 20 and 21 depict two embodiments of staggered pixel structures fora sensor array, in accordance with embodiments of the present invention;

FIG. 22 is a perspective schematic diagram of a sensor and a spectrumand a shifted spectrum impinging on the sensor, according to oneembodiment of the present invention;

FIG. 23 is a graph illustrating a known spectrum; and

FIG. 24 is a flowchart that describes automatic wavelength calibration,according to embodiments of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In accordance with embodiments of the present invention, methods andapparatus are disclosed for analyzing composition of a sample with ahand-held, self-contained, battery-powered test instrument. Aspectrometer in the test instrument has a wavelength range broad enoughto enable the test instrument to detect and determine relativequantities of carbon, phosphorous, sulfur, manganese, silicon, iron andother elements necessary to identify common alloys. The testinstrument's design and construction enables the test instrument toproduce accurate results over a wide ambient temperature range, withoutheating or cooling the spectrometer, thereby conserving power andextending the amount of time the test instrument may be operated beforethe battery needs to be recharged.

The test instrument excites at least a portion of a sample, thusproducing an optical signal. As a result of optical emissions atwavelengths characteristic of elements in the sample, the optical signalcontains information that identifies the elements in the sample. Thespectrometer wavelength-disperses the optical signal onto a set ofsensors, each of which receives a narrow range of wavelengths of theoptical signal. A processor is programmed to receive and process signalsfrom the sensors and to identify and quantify the elements in thesample.

A hand-held, self-contained, battery-powered test instrument should besmall, light-weight and consume little electrical power. Disclosedembodiments of the present invention enable construction ofspectroscopy-based analytical test instruments that exhibit theseproperties. These embodiments are discussed in the context of analyticaltechniques and test instruments that employ optical emissionspectroscopy (OES); however, the teachings of this application areapplicable to other types of analytical test instruments that employspectral analysis, including test instruments that employ opticalabsorption spectroscopy. Furthermore, although the disclosed embodimentsare discussed in the context of arc/spark excitation, other forms ofexcitation, including laser-induced breakdown (LIB) and glow discharge(GD) may be used.

General Structure of One Embodiment

FIG. 1 is a perspective view of a hand-held, self-contained,battery-powered OES test instrument 100, according to one embodiment ofthe present invention. The instrument 100 includes a snout 102. Inoperation, an electrically-conductive flat portion 103 of the snout 102is pressed against an electrically-conductive sample surface (notshown). A spark from a counterelectrode 104 to the sample excites aportion of the sample, thereby producing an optical signal. Thecounterelectrode 104 is electrically insulated from the electricallyconductive flat portion 103 of the snout 102, such as by an insulateddisk (not visible). The optical signal enters an upper port (notvisible) and is reflected by one or more mirrors (not visible) into aspectrometer 204 inside the instrument 100. A processor (not visible) iscoupled to a set of detectors (not visible) in the spectrometer. Theprocessor is programmed to process signals from the detectors. Theprocessor analyzes at least a portion of the spectrum produced by thespectrometer to identify and quantify the elemental composition of thesample.

The processor displays results of the analysis on a touchscreen 110.Optionally, the processor may transmit results of the analysis to anexternal device, such as a computer or display, via a wired or wirelessconnection (not shown). The touchscreen 110, a trigger 112 and operatorinterface buttons 114 enable a user to interact with the processor. Adetachable, rechargeable battery 116 powers the processor, touchscreen110, spectrometer 204 and a spark generator (not visible) that iscoupled to the counterelectrode 104.

FIG. 2 is a cut-away view of the test instrument 100 showing the sparkgenerator 200, a first mirror 202 and the spectrometer 204. The mirror202 may be an aluminized front-surface mirror with a magnesium fluoridecoating and a fused silica substrate, although other suitable mirrorsmay be used. The mirror 202 may be planar, although a concave (includinghyperbolic or parabolic) shape may provide better image quality.

An electrically conductive insert 206 defines a bore 208, in which thecounterelectrode 104 (not shown in FIG. 2) is disposed. The insert 206also provides at least part of the electrically conductive flat portion103 of the snout 102. The spark generator 200 is electrically connectedto the counterelectrode 104 and to the electrically conductive flatportion 103 of the snout 102 to complete on an electrical return circuitwith the sample, when the flat portion 103 of the snout 102 is broughtinto contact with the sample. Much of the snout 102 may be metal oranother heat-conductive material to dissipate heat from the spark andfrom the spark generator 200. A dashed line 210 schematicallyillustrates a portion of a light path (largely hidden within the snout102) taken by the optical signal from the vicinity of thecounterelectrode 104 to an entrance slit of the spectrometer 204.

FIG. 3 is a perspective view of the spectrometer 204. The spectrometer204 housing includes a structure 300 that defines an opening 302, intowhich the insert 206 (not shown in FIG. 3) in inserted. FIG. 4 is aclose-up perspective view of a portion of the spectrometer 204 showingthe structure 300 and the insert 206 in more detail. The insert 206defines a bore 400 through a wall of the insert 206, as shown by dashedlines. The optical signal produced by the sample passes through the bore400 and is reflected by a second mirror 402 in the structure 300. Thesecond mirror 402 reflects the optical signal via a second bore 404through the structure 300 to the first mirror 202 (not shown).

A third bore 406 in the structure and a corresponding bore (not visible)in the insert 206 provide a fluid communication path through which a gasmay be plumbed to the vicinity of the counter electrode 104 to purge thevicinity of the counterelectrode 104 of air, at least in part becausethe air may attenuate or block some or all wavelengths of interest in anoptical signal. A window (not shown) mounted in the bore 404 provides agas-tight seal to prevent the gas from escaping from the bore 208 intothe spectrometer 208. The window is preferably made of beta alumina(β-Al₂O₃ or “synthetic sapphire”) or another material that issufficiently transparent at the wavelengths of the optical signal.

Environment

As shown in FIG. 3, a sharpened tip portion of the counterelectrode 104is disposed about 2-3 mm from the sample surface 500, thereby creatingan analytical gap. The counterelectrode 104 may be about 1/16-¼ inch indiameter. The counterelectrode 104 is preferably made of thoriatedtungsten, although other suitable materials, such as carbon (graphite)or silver may be used. The counterelectrode 104 should be made of amaterial that produces a simple spectrum if excited, or at least aspectrum that is easily distinguished from spectra produced by likelymaterials in the sample 500.

An inert gas, such as argon, may be plumbed via the bore 406 to floodthe analytical gap with the gas. Methods and apparatus for providing agas to a hand-held test instrument, including from a gas tank coupleddirectly to, and possibly enclosed within a portion of, the instrument,are disclosed in detail in Provisional Patent Application No.60/889,465, filed Feb. 12, 2007, titled “Small Spot X-Ray Fluorescence(XRF) Analyzer,” the contents of which are incorporated by reference asif fully set forth herein. A gas that is not chemically reactive withlikely materials in the sample 500, and that produces a relativelysimple emission spectrum when excited (or at least a spectrum that iseasily distinguished from spectra produced by likely materials in thesample 500), should be selected.

Spark Generation

An electrical potential between the counterelectrode 104 and the samplesurface 500 breaks down the gas, enabling an electrical current, in theform of a spark or an arc or both, to flow from the counterelectrode 104to the sample surface 500. The spark heats the gas and vaporizes a smallamount of the sample. The vaporized sample material is excited by thehot gas and produces an optical (although possibly invisible) discharge.

Positive unidirectional current should be provided to thecounterelectrode 104 to prevent eroding the counterelectrode 104. Thespark generator 200 includes a diode 508 (or an equivalent circuit) toprovide an appropriate unidirectional current to the counterelectrode104. The counterelectrode 104 may be cleaned of debris buildup with awire brush or by reversing the current and producing sparks/arcs to asacrificial cleaning sample.

In operation, a series of sparks/arcs may be generated in rapidsuccession. Each spark may strike a slightly different location on thesample surface 500, due to pitting of the sample surface 500 by thesparks, imperfections and inclusions in the sample surface 500, etc. Ingeneral, a high spark repetition rate causes the sparks to strike thesample surface over a smaller area than a low spark repetition ratecauses. Thus, it is possible to control the sample area by controllingthe spark repetition rate. At about 50 to 400 sparks per second, thesparks strike an area about 3 mm in diameter, whereas at about 1,000 to2,000 sparks per second, the discharge area is about 1 mm in diameter.It may be desirable to avoid small discharge areas, such as 1 mm indiameter, at least in part because most metals are not sufficientlyhomogeneous to yield accurate results when only such a small area istested. Sampling such a small area may produce a result that is biasedby the composition of the small area.

Voltage and current (versus time) profiles (waveforms) of a signalprovided to the counterelectrode 104 to produce the spark/arc should becontrolled to optimize initiating the spark, vaporizing the sample andheating the gas, while limiting power consumption. FIG. 6 is a graphshowing representative voltage and current curves plotted against timefor a single spark/arc. As shown in the graph, a short-duration,high-voltage peak 600 initiates a spark to breakdown the gas in ananalytical gap between the counterelectrode 104 and the sample surface500. The spark erodes a portion of the sample into the analytical gas.Thereafter, the voltage is reduced. The spark is a low-current spark, asindicated in a portion 602 of the current graph. However, thereafter thecurrent is increased and reaches a peak 604 while the voltage ismoderately high to sustain an arc to excite the eroded sample materialin the analytical gap. The excited material emits an optical signalcharacteristic of the elemental composition of the excited material.Thereafter, the current and voltage are reduced. Due to the varyingamounts of power introduced into the analytical gap over the course ofthe spark/arc, the temperature in the analytical gap varies over theduration of the spark/arc.

For analyzing hard metals, such as iron and nickel, a voltage profile asshown in the top graph (A) of FIG. 7 may be used. The voltage profileshows a high-energy pre-spark 700, followed by a high-current, but lowervoltage, arc 702. For analyzing soft metals, such as aluminum, magnesiumor copper, a two-phase voltage profile as shown in the bottom graph (B)of FIG. 7 may be used. The voltage profile shows a spark portion 704, adelay 706 and a separate arc portion 708. The spark portion 704 may beused to determine the primary alloy in a sample, and the arc portion 708may be used to determine trace elements in the sample. In general, thevoltage of the signal in the lower graph (B) is less than the voltage ofthe signal in the upper graph (A).

In one embodiment, the initial breakdown voltage is about 6,000-10,000volts, as required to break down the argon or other gas in theanalytical gap. In one embodiment, the peak current is about 60-100amps. The absolute values of the voltages and currents are not asimportant as repeatability of the voltages and currents from spark/arcto spark/arc and avoiding ringing in the signal to the counterelectrode104. The amplitudes and profiles of the voltages and currents should beas repeatable as practical.

The spark generator 200 operates under the control of the processor.That is, the processor may specify a repetition rate, as well asvoltages, currents and/or profiles, to the spark generator 200.Alternatively, the voltages, currents and/or profiles may bepre-configured in the spark generator 200. The spark generator 200 maybe any suitable circuit, such as a switched-mode power supply (SMPS),such as high-power thyristor or MOSFET circuit, that produces voltagesand currents as described above.

To prevent accidental exposure of a user to spark voltage, the snout 102may include one or more momentary contact switches, pressure transducersor other sensors that must be activated by a sample surface before thespark generator 200 produces a spark signal. One embodiment of such asafety interlock system is shown in FIG. 1. Three momentary contactswitches 120, 122 and 124 are mounted in the snout 102, such that theswitches 120-124 are activated only if the flat portion 103 of the snout102 is fully engaged against the surface of a sample.

Light Collection

Referring back to FIG. 5, it should be noted that the light path 502forms an angle 504 with the sample surface 500. FIG. 8 is a close-upview of the analytical gap, showing a discharge region 800 between thecounterelectrode 104 and the sample surface 500. The portion of theregion 800 closer to the sample surface 500 is hotter (at about 30,000°C.) than the portion of the region 800 (at about 1,500° C.) closer tothe counterelectrode 104. Thus, hard line emissions 802 from elementssuch as phosphor, sulfur and carbon emanate from the hotter portion ofthe region 800. Conversely, soft line emissions 804 from elements suchas aluminum and copper emanate from the cooler portion of the region800.

Emissions from an analyte should be sampled from a volume of theanalytical gap where the analyte is ionized. The hard line emissions 802should be observed at an angle of about 1-5°, whereas the soft lineemissions 804 should be observed at an angle of about 3-15°. An angle ofabout 3° provides a good compromise, enabling observation of both thehard line emissions 802 and the soft line emissions 804. Referring againto FIG. 5, the angle 504 may be about 3°, although other small anglesmay be used. In addition, multiple optical paths, possibly each at adifferent angle, may be provided from the analytical gap and recombinedcloser to the spectrometer 204. A mask 806 should be used to avoidobserving emissions from the hot tip of the counterelectrode 104 oremissions from the sample surface 500.

As noted above, a combination of high-energy and low-energy sparksand/or arcs may be used in a series of excitations, facilitatingdetecting hard metals and soft metals in a sample during differentsparks/arcs. In one embodiment, the major element(s) in the sampleis(are) analyzed with a first pulse, and trace elements in the sampleare analyzed with a second pulse.

Emissions from some elements peak later during a spark/arc thanemissions from other elements. Similarly, “background” emissions, suchas from the sample surface 500 or the tip of the counterelectrode 104,may peak earlier than emissions from some of the elements in the sample.For example, emissions from lead peak late, after many of the backgroundemissions have subsided. Time-resolved analysis of the optical signalmay provide a better signal-to-noise ratio by analyzing the spectrum foremissions from particular elements when those emissions peak.Time-resolved analysis of an optical signal is discussed in detail inprovisional patent application No. 60/891,320, filed Feb. 23, 2007,titled “Time-Resolved Spectroscopy with Sensor Array” and in U.S. patentapplication Ser. No. 12/035,477, by Denis Baiko, et al, titled “Fast andPrecise Time-Resolved Spectroscopy with Linear Sensor Array,” filed Feb.22, 2008, the contents of which are incorporated by reference as iffully set forth herein.

Spectrometer

Maintaining physical relationships, such as distances and orientations,among optical components of a spectrometer is necessary to maintainaccuracy of the spectrometer. In a traditional spectrometer, the opticalcomponents, such as a structure defining an entrance slit, a diffractiongrating and one or more sensors, are rigidly mounted to a structuralmember made of cast iron or Invar (FeNi), and the spectrometer istemperature controlled to limit thermal expansion or contraction of thestructural member. Temperature control is traditionally achieved byheating the spectrometer to a uniform and constant temperature, althoughsome spectrometers are cooled, rather than heated. In either case,energy is consumed to heat or cool the spectrometer. Sometimes electricfans are used to circulate air within or around a spectrometer tomaintain a uniform temperature. Heating a spectrometer may necessitateselecting temperature-insensitive sensors or cooling sensors within thespectrometer to avoid generating heat-induced noise in the sensors.

A spectrometer in a hand-held test instrument should be small,light-weight and consume little electrical power. FIG. 9 is a cutawayperspective schematic diagram (with the cover removed for clarity) ofthe spectrometer 204, according to one embodiment of the presentinvention. Various aspects of the spectrometer 204, including itscross-dispersed design, contribute to its compact size, light weight andlow power consumption.

Structural components, such as the case 900 and cover (removed forclarity), of the spectrometer 204 are made of a light-weight material,such as graphite-filled polyphenylene sulfide (PPS), that has a smallcoefficient of thermal expansion (CTE) over a range of expected ambienttemperatures in contexts where the test instrument 100 may be used. Thesmall CTE reduces or eliminates the need to temperature-control thespectrometer 204, thereby conserving electrical power, while maintainingthe accuracy of the spectrometer 204. In addition, PPS is black, whichassists in absorbing stray light within the spectrometer 204. PPS may bemachined or injection molded, or a combination thereof, to produce thestructural components of the spectrometer 204.

PPS is available from Chevron Phillips, The Woodlands, Tex., under thetradename Ryton PPS. Polyphenylene sulfide filled with about 40%graphite is preferred. Such a material is available under thedesignation IPC-1834 from Hoerbiger America Rings & Packing, Inc.,Houston, Tex. 77023 or under the designation “Bearing Grade” fromBoedeker Plastics, Inc., Shiner, Tex. Other polymers, high-carboncomposites, glass-filled polymers or liquid crystal polymers thatexhibit or are modified, such as by filling with carbon or anothersuitable filler, for a small CTE at expected ambient temperatures mayalso be used.

The optical signal 210 is reflected by the mirror 202 onto an entranceslit 901. The entrance slit 901 may be about 5 μm wide. A prism 902vertically disperses the incoming light, as indicated at 904. The prism902 is located about 60 mm from the entrance slit 901. The prism 902 ispreferably made of beta alumina or another material that is sufficientlytransparent at the wavelengths of interest.

The prism 902 is attached to a field stop (internal baffle) 906. Thefield stop 906 and prism 902 are shown enlarged and from another view inthe insert in FIG. 9. The prism 902 should be mounted at its minimumdeviation angle to minimize astigmatism. In one embodiment, the prism902 refracts light at an angle of about 6.8°, thus the prism 902 istilted at about half that angle, so the axis of the output from theprism 902 (toward the grating 910) is parallel to the floor 905 of thespectrometer case 900. The back 907 of the field stop is angled to tiltthe prism 902 appropriately.

The field stop 906 defines an approximately ¼-inch wide aperture 908,through which the vertically-dispersed light passes. As noted, sparkstrikes on the sample surface 500 occur within a small area, notnecessarily at a single point on the surface. To accommodate this spark“wander,” the image of the spark is defocused somewhat at the entranceslit 901; instead, the image is focused on the internal baffle 906.

The vertically-dispersed light 904 from the prism 902 impinges on aconcave holographic grating 910. The grating 910 is about ½-inch thickand about 50-75 mm in diameter. The internal baffle 906 masks off theedges of the discharge volume in the analytical gap, thus preventing anoptical signal from the tip of the counterelectrode 104 or from thesample surface 500 from reaching the grating 910. The grating 910horizontally disperses the light. The horizontally-dispersed lightimpinges on an array of sensors 912. The grating 910 is constructed tohave comparable efficiencies in two different, although not necessarilyconsecutive, orders. Each order may be positive or negative.

The grating 910 produces two distinct spectra, which will be referred toas a first-order spectrum 914 and a second-order spectrum 916, on thesensors 912. The resolution of the second-order spectrum 916 may begreater than the resolution of the first-order spectrum 914. Because theprism 902 vertically disperses the incoming light 900, long and shortwavelengths of the vertically-dispersed light impinge on the grating 910at different angles. This angular difference causes a verticaldisplacement 918 between the first-order spectrum 914 and thesecond-order spectrum 916 on the sensors 912. In one embodiment, thevertical displacement 918 is about 2 mm. The sensors 912 may include tworows of sensors, one row of sensors for each spectrum 914 and 916,although an alternative embodiment of the sensors 912 is describedbelow.

Order Separation

Various system parameters influence the extent of the verticaldisplacement 918. If an insufficient amount of vertical displacement 918is provided, the two order spectra 914 and 916 partially or completelyoverlap each other on the sensors 912. Depending on the heights of thesensor pixels, such an overlap may make it impossible to achieve a cleanspectrum on each set of sensors. The upper portion of FIG. 10schematically represents two order spectra 1000 and 1002, as imaged onthe sensors 912 (FIG. 9), in which the two spectra 1000 and 1002significantly overlap, possibly preventing achieving a clean spectrum oneach set of sensors.

Returning to FIG. 9, the amount of vertical displacement 918 between thetwo spectra 914 and 916 on the sensors 912 depends, in part, on theamount of dispersion 904 caused by the prism 902 which, in turn, dependson the index of refraction of the material of the prism 902 and on theapex angle of the prism 902. However, large apex angles result inthicker prisms, which may further attenuate the optical signal,particularly in the ultraviolet range.

The vertical displacement 918 also depends, in part, on the linearmagnification of the focusing system (i.e., on magnification of theconvex grating 910), on the distance 920 between the prism 902 and thegrating 910 and on the distance between the grating 910 and the sensors912. The grating 910 is a focusing element, thus decreasing theseparation 920 increases the displacement 918. In one embodiment, thelinear magnification of the convex grating 910 is about −1.

In one embodiment, the prism 902 is placed as close as possible to thegrating 910, without occluding the light path, between the grating 910and the sensors 912, of any portion of either order's spectrum 914 or916 that is of analytical interest. Such a placement of the prism 902 inthis embodiment creates a slit 901 to prism 902 distance of about 60 mm.The lower portion of FIG. 10 is a schematic diagram of two order spectra1004 and 1006, as imaged on the sensors 912 (FIG. 9), achieved bypositioning a beta alumina prism having an apex angle of about 8° asdescribed above. As can be seen in lower portion of FIG. 10, the twospectra 1004 and 1006 do not overlap and provide a vertical displacementof about 2 mm between the spectra 1004 and 1006, although thedisplacement may vary with wavelength. In one embodiment, thedisplacement varies from about 0.6 mm to about 3 mm over the wavelengthrange of interest.

For identifying ferrous and other common metals, optical emissions fromthe analytical gap that have wavelengths between about 170 nm and about410 nm are of interest. In one embodiment, the grating 910 (FIG. 9) isconstructed such that the efficiency of the grating at the first orderis relatively high for wavelengths between about 247 nm and about 410 nmand relatively low outside this range, and the efficiency of the gratingat the second order is relatively high for wavelengths between about 170nm and about 247 nm and relatively low outside this range, althoughthere may be some overlap between the high-efficiency portions of thefirst and second orders. The grating 910 design, the cross-dispersionprovided by the combination of the prism 902 and the grating 910 and thetwo rows of sensors 912 enable the spectrometer 204 to analyze arelatively broad range of wavelengths in a relatively small amount ofspace. In some embodiments, there may be a spectral gap or overlapbetween the first-order spectrum 914 and the second-order spectrum 916on the sensors 912.

The vertical displacement 918 between the first-order spectrum 914 andthe second-order spectrum 916 may be insufficient for two co-planar rowsof sensors 912. In this case, one of the two rows of sensors may beoriented in a plane that is perpendicular to the sensor array 912 shownin FIG. 9, and a mirror may be used to reflect one of the two spectraonto the perpendicular sensor array. A side view of such an arrangementis shown schematically in FIG. 11, and a perspective view (lookingslightly upward and from the side) of such an arrangement is shown inFIG. 12. Referring to FIG. 11, a mirror 1100 reflects first order light1102 to a downward-facing row of sensors 1104. Second order light 1106impinges directly, i.e. without first being reflected, on aforward-facing row of sensors 1108. At the wavelengths of interest,mirrors reflect longer wavelengths of light more efficiently thanshorter wavelengths. For example, below about 240 nm, about 20% of anoptical signal is lost as a result of reflecting the signal with amirror. The order primarily composed of wavelengths that are lessefficiently reflected by a mirror should impinge on the forward-facingrow of sensors 1108. As noted, in one embodiment, the second order light(ranging from about 170 nm to about 250 nm) impinges directly on theforward-facing sensors 1108. FIG. 12 shows mounting brackets 1200 usedto mount the mirror 1100 and the two rows of sensors 1104 and 1108 tothe floor 905 of the housing 900.

Spectral resolution is generally defined as the spectral separationbetween the two closest peaks that a spectrometer can resolve. For adigital sensor that includes a set of adjacent pixels to resolve twopeaks, at least one pixel between the two peaks should receive a lowersignal than its neighbors, as shown schematically in FIG. 19. If thepeaks fall on the sensor so that the pixels with the maximum signals arenext to each other, the two peaks may not be resolved by thespectrometer/sensor combination.

Spectrometer bandpass (BP) specifies how much spectral bandwidth is seenat a given wavelength position. Since bandpass limits the ability of aspectrometer to separate peaks, it is common to refer to the BP as thespectral resolution of the spectrometer. The BP may be calculated fromthe output image width and the reciprocal linear dispersion of thedispersive element in the spectrometer. The reciprocal linear dispersionindicates the width of spectrum that is spread over a distance of 1 mmat the focal plane, i.e., the sensors 912 in the description above.Reciprocal linear dispersion, which varies with wavelength, is given innm/mm and is typically listed as a primary instrument specification. Thereciprocal linear dispersion of the diffraction grating depends largelyon the pitch of the grooves in the grating.

In one embodiment of the spectrometer 204, the diffraction grating 910has a reciprocal linear dispersion of about 5 nm/mm, and the entranceslit 901 has a width of about 5 μm. Thus, in one embodiment, thediffraction grating 910 provides a resolving power of at least about5,000 and, in another embodiment, at least about 10,000. In oneembodiment of the spectrometer 204, each sensor 1004 and 1008 has aneffective pixel pitch of about 7 μm. The resulting resolution is about0.02 nm per pixel in the second-order spectrum 916 and about 0.04 nm perpixel in the first-order spectrum 914.

Each sensor 1104 and 1108 may incorporate two or more rows of pixels inwhich the pixels are staggered horizontally, which increases theeffective optical resolution of the device. FIGS. 20 and 21 depict twoalternate staggered pixel configurations. A typical pixel 2000 includesa light-sensitive area 2002 and a surrounding light-insensitive area2004. In a typical spectroscopic application, such as spark OES, theoptics are configured to impose a tall narrow slit image 2006 or 2100onto the detection device. By manufacturing the device with two or morerows of pixels, each with a horizontal resolution of X, where the pixelsin the other row(s) are offset by a distance of X/2, the effectiveresolution of the system improves to X/2. This, in turn, allows for theuse of more narrow entrance slits, which effectively improves thespectroscopic resolution of the system.

One embodiment of the present invention operates with uncollimatedoptical signals provided to the prism 902. Although uncomimated signalsmay cause a small amount of aberration in the image projected on thesensors 1104 and 1108, any “smearing” of the image is generally in thesame direction as the long dimension of the rectangular pixels. Thus,these aberrations do not negatively affect the resolution of thespectrometer. In addition, using lower orders, such as first and second,of diffracted signals from the grating 910 may minimize someaberrations.

The spectrometer described herein may be used in applications other thanhand-held analytical instruments. For example, the spectrometer may beused in bench-top analyzers, telescopes, telecommunications equipment,etc.

Dynamic Wavelength Calibration

In one embodiment, each row of sensors 1104 and 1108 contains about4,096 pixels; however, in other embodiments other numbers of pixels maybe used. Sensors that include more pixels than are necessary to image aspectrum may provide advantages. FIG. 22 is a perspective schematicdiagram of a sensor 2200 and a spectrum 2202 impinging on the sensor2200. The sensor 2200 contains a row 2204 of sensor pixels, exemplifiedby pixels 2206, 2208 and 2210. If the row of pixels 2204 is just wideenough to capture a spectrum of analytical interest, then when thesensor is mounted in the spectrometer, the position of the sensor 2200(or another component, such as the grating) may need to be carefullyadjusted, so the entire spectrum is imaged, i.e., the entire imageimpinges on the row of pixels 2204.

However, if the row of pixels 2204 is longer than the spectrum 2202 iswide (as shown in FIG. 22), the sensor 2200 may be mounted with lesspositional precision, as long as the entire spectrum 2202 fallssomewhere on the row of pixels 2204. Essentially, the additional pixels,i.e., the number of pixels in excess of the number needed to image theentire spectrum 2202, provide a tolerance, within which the sensor 2200may be mounted. Once the sensor 2200 is mounted, the sensor 2200 or theprocessor (not shown) may determine which pixels are illuminated by thespectrum 2202 and, if desired, assign pixel numbers or addressesbeginning with the pixel at one end of the spectrum 2202. If thespectrum 2202 shifts position on the sensor 2200, as indicated byspectrum 2212, due to, for example, thermal expansion or contraction ofa component of the spectrometer or elsewhere in the instrument, thesensor 2200 or the processor may compensate by renumbering the pixels orreading data from a different set of pixels, corresponding to thelocation where the spectrum 2212 has shifted.

In one embodiment, the optical system and sensor is configured such thatthe sensor detects a first order spectrum that extends from about 246.9nm to about 410 nm, and the sensor detect a second order spectrum thatextends from about 170 nm to about 246.9 nm. There should be someoverlap between the two spectra at, for example, 246.9 nm.

The row of pixels 2204 may be wavelength calibrated, i.e., the pixelsmay be associated with wavelengths, by testing a sample that has a knowncomposition and correlating expected peaks in the spectrum with pixelsthat experience correspondingly high values of illumination. In oneembodiment, the processor automatically wavelength calibrates the row ofpixels 2204 by matching an observed spectral feature with one of a setof stored feature prototypes. Essentially, the processor matches thepattern of the observed feature with a known pattern.

The pattern may include relative spacing between or among peaks, valleysor other spectral characteristics and relative height(s) of the peak(s),valley(s), etc. For example, FIG. 23 illustrates a known spectrum. Thespectrum contains well-defined peaks for various elements. One or morestored feature prototypes are stored in a memory accessible by theprocessor. The feature prototypes need not include information about anentire spectrum of a material; the prototype may include informationabout only selected peaks, etc.

Prototypes may be based on expected “matrix” elements in expectedsamples, because these elements will likely have a strong presence inevery sample exposure. For example, for iron and steel samples, aprototype that contains information about elemental iron (Fe) may beused, and for aluminum alloys, a prototype that contains informationabout elemental aluminum (Al) may be used.

After the instrument takes a reading, the processor searches the dataprovided by the sensors for a match with one or more of the storedprototypes. Note that the data from the sensor may include signatures ofadditional materials that are included in the tested sample, but are notrepresented in the prototypes. The prototypes may be chosen so thattheir patterns are easily detected among other likely materials insamples. For initial wavelength calibration, a known standard may beused as the sample.

Once the processor identifies a prototype pattern that matches observeddata, the processor associates one or more pixels, where one or morefeatures of the prototype are observed, with corresponding wavelength(s)stored with the prototype data. In one embodiment, the processorassociates a wavelength or wavelength range to one pixel, according toan observed and matched feature, and assigns other wavelengths or rangesto the other pixels based on an expected linear spectral dispersionbased on the geometry of the spectrometer.

In another embodiment, the processor associates a wavelength or range toa pixel, as described above, and calculates an actual linear spectraldispersion observed on the sensor, based on relative spacing between oramong observed and matched features, and associates wavelengths orranges with other pixels, based on the calculated linear spectraldispersion.

The wavelength calibration may create a mapping between pixel number andwavelength. The dispersion is not necessarily constant across the lengthof the detector. Thus, identifying more peaks allows for a higher ordermapping function to be used. For example, identifying one peak allowsfor a 0th order “shift” correction, two peaks allows for a 1st orderlinear correction, and so on.

FIG. 24 is a flowchart that describes automatic wavelength calibration.At 2400, a reading is taken, i.e., a spectrum from a sample impinges onthe sensors, and the processor reads at least some of the sensors. At2402, a search is conducted of the observed spectrum and stored spectralfeature patterns for a pattern that matches at least a portion of theobserved spectrum. At 2404, a wavelength or range of wavelengths(hereinafter collectively referred to in this context as a wavelength)is associated with a pixel (such as the first pixel in the sensorarray), based on a correspondence between a second pixel that is locatedat the centroid of a first observed spectral feature (such as a featurein the matched prototype) and information (such as wavelength) about thespectral feature pattern. This information may be stored in a memory.

In one embodiment, other wavelengths are assigned to other pixels basedon an expected linear spectral dispersion based on the geometry of thespectrometer.

In another embodiment, at 2406, a linear spectral dispersion on thesensor is calculated, based on a correspondence between another pixel atthe centroid of a second observed spectral feature and information (suchas wavelength) about the second spectral feature and the number ofpixels (or distance) between the two pixels at the centroids of theobserved features. At 2408, a wavelength is associated with otherpixels, based on the wavelength associate with the pixel at 2404 and thecalculated linear spectral dispersion.

The operations at 2404 and 2406 may, collectively, associate wavelengthswith all the pixels in the sensors. On the other hand, these operationsmay associate wavelengths with only a portion of the pixels. In thatcase, as indicated at 2410, necessary operations may be repeated forother groups of pixels.

Thus, the disclosed instrument may be more easily assembled orsubsequently adjusted, without requiring high precision positionaladjustments to the sensors. In addition, the instrument may maintain itsaccuracy over time and in the face of temperature-induced dimensionalchanges, imperfect imaging of the wandering spark source, mechanicalvibration, physical shock and the like by dynamically wavelengthcalibrating itself, based on observed spectral features. This wavelengthself-calibration may be performed automatically at the beginning of eachsample run or after a predetermined number of runs, at otherautomatically determined times (such as between sparks or afterdetection of a physical shock by an accelerometer or a temperaturechange by a thermistor within the instrument) or in response to auser-entered command.

Diffraction Grating Mount Assembly

FIG. 13 shows an exploded view of a diffraction grating assembly 1300.In the diffraction grating assembly 1300, the diffraction grating 910 isheld between a compression ring 1301 and a diffraction grating mount1302. Immediately behind the grating 910 is a thin (approximately1/32-inch thick) elastomeric pad 1304, which may be cork or anothersuitable material. A grating compression plate 1306 is disposed betweenthe elastomeric pad 1304 and the diffraction grating mount 1302. Thecompression ring 1301, the compression plate 1306 and the diffractiongrating mount 1302 are preferably made from the same material as thecase 900 of the spectrometer 204. The compression ring 1301 is attachedto the diffraction grating mount 1202 by two screws (one of which isshown at 1307), which pass through holes 1308 and 1310 in thecompression ring 1301 and thread into corresponding holes (one a whichis visible at 1312) in the diffraction grating mount 1302. Thecompression ring 1301 applies even pressure along the perimeter of thediffraction grating 910, and the elastomeric pad 1304 enables thediffraction grating 910 to expand or contract with temperature changeswithout distorting the diffraction grating 910.

The diffraction grating assembly 1300 is mounted in the diffractiongrating housing 900, as shown in FIG. 14. Adjustment screws 1400 and1402 may be used to tilt the diffraction grating assembly 1300. A well1404 in the floor of the housing 900 provides clearance for the bottomportion of the compression ring 1301. Preferably, the diffractiongrating 910 is tilted about 4° back from normal to the floor of thehousing 900. Light from the prism 902 impinges on the diffractiongrating 910 at an upward angle. Tilting back the diffraction grating 910causes the dispersed light to follow a path to the sensors 912 that isapproximately parallel to the floor of the housing 900.

Test Instrument Alignment

It may be necessary to align the optics of the test instrument 100. Themirror 202 may be aligned using a setup illustrated in FIG. 15. Thecounter electrode 104 is removed from the test instrument 100 and afiller block 1500 may be inserted from the back of the snout 102 totemporarily replace the counter electrode assembly. A tubular mask 1502,as illustrated by front and top news in the insert in FIG. 15, isinserted into the opening in the front of the snout 102. An ultraviolet(UV) light source 1504 is inserted into the opening in the front of thesnout 102 and into the mask 1502, leading approximately 4 mm of the UVlight source 1504 exposed within the opening in the front of the snout102. The tubular mask 1502 is sized to accommodate the outside diameterof the UV light source 1504 and the inside diameter of the opening inthe front of the snout 102. Additional shielding 1506 may be used tominimize UV leakage. The mirror 202 is then adjusted to maximize theamplitude of the signal 1508 reaching the spectrometer 204 (not shown).

Alternatively or in addition, a visible or invisible laser beam may beintroduced into the spectrometer 204 in the vicinity of the sensors 912and projected backwards through the spectrometer 204 to the spark gap. Aport (not shown) may be provided in the housing 900 of the spectrometer204 to facilitate introducing the laser beam. Optical components in thespectrometer 204 and the mirror 202 may be adjusted until the laser beamis detected in the opening in the front of the snout 102 where analyteemissions are expected to be produced. An expected path of the laserbeam through the spectrometer may be calculated, based on the wavelengthof the laser beam. It should be noted that the path taken by the laserbeam toward the diffraction grating 910 may not coincide with the pathstaken by some wavelengths of the optical signal dispersed by thediffraction grating 910 towards the detectors 912, due to the wavelengthof the laser beam and the angle at which the diffraction grating 910reflects light at that wavelength. Thus, sensors 912 may be clear of theport by which the laser beam is introduced into the spectrometer 204.

Further alignment may be performed by reflecting the laser beam at theopening in the front of the snout 102, back along the optical path, tothe spectrometer. Alternatively or in addition, a laser beam may beintroduced into the opening in the front of the snout 102 and directedalong the optical path to the spectrometer.

FIG. 16 is a flow chart that describes a process for analyzingcomposition of a sample. At 1600, an environment in which a portion ofthe sample may be analyzed is created. Creating this environment mayinclude purging air from the portion of the sample that is to beanalyzed. An inert gas, such as argon, may be used to purge the air.

At 1602, a portion of the sample is excited. The sample may be excitedwith an electric spark/arc, a laser, glow discharge or another suitablemechanism. If an electric spark/arc is used, a spark gap is createdbetween a counterelectrode and the sample. The counterelectrode and thesample are electrically connected to a spark source, which produces asuitable potential having an appropriate waveform. A potentialdifference between the counterelectrode and the sample breaks down thegas in an analytical gap and erodes a portion of the sample into theanalytical gap. The potential may be reduced, and current may beincreased, to ionize the sample material in the analytical gap. Theionize sample material emits an optical signal.

At 1604, the optical signal is collected. The optical signal is routed,via an optical path, to a spectrometer. At 1606, the optical signal iswavelength-dispersed. The optical signal may be cross-dispersed. At1608, the intensity of the dispersed optical signal is measured atwavelengths of interest. One or more arrays of sensors may be used tomeasure the intensities of the dispersed optical signal. If the opticalsignal is cross-dispersed, one set of the sensors may be disposed adistance away from the other of the sets of sensors, according to theamount of cross-dispersion. At 1610, the intensity measurements areprocessed to determine the composition of the sample. The processing maybe performed by a processor executing instructions stored in a memory.The process may be repeated, as indicated 1612, for a series ofmeasurements. Data from the series of measurements may be averagedand/or parameters of the excitation (1602) may be varied for each of therepetitions.

FIG. 17 is a block diagram of major components of the test instrument100. Instructions for a processor 1700, as well as spectral featureprototypes, may be stored in a memory 1702. Analytical results fromsamples may also be stored in the memory 1702 and displayed on thetouchscreen 110 and/or provided to an external device via a wired orwireless data port 1704. In addition, the memory 1702 may store tablesof compositions of known materials (such as alloys) for comparison tocompositions of test samples, and results of this comparison may bedisplayed on the screen 110 and/or provided via the port 1704.

Referring to FIG. 1, the touchscreen 110 is readable while the testinstrument 100 is in most orientations. However, in some cases, thetouchscreen may be difficult to read. A hinged (tilt-up) screen may beused in some OES, x-ray fluorescence (XRF) or other of hand-held,self-contained test instruments. One embodiment of such a tilt-up screenis shown at 1800 in FIG. 18. A flexible ribbon cable or other suitableflexible wire is used to connect the screen 1800 to the processor orother circuitry within the test instrument 100.

Although a spectrometer having a wavelength range of about 170 nm toabout 410 nm has been described, spectrometers according to the presentinvention may have other wavelength ranges.

A hand-held, self-contained, battery-powered test instrument has beendescribed as including a processor controlled by instructions stored ina memory. The memory may be random access memory (RAM), read-only memory(ROM), flash memory or any other memory, or combination thereof,suitable for storing control software or other instructions and data.Some of the functions performed by the test instrument have beendescribed with reference to flowcharts. Those skilled in the art shouldreadily appreciate that functions, operations, decisions, etc. of all ora portion of each block, or a combination of blocks, of the flowchartsmay be implemented as computer program instructions, software, hardware,firmware or combinations thereof. Those skilled in the art should alsoreadily appreciate that instructions or programs defining the functionsof the present invention may be delivered to a processor in many forms,including, but not limited to, information permanently stored onnon-writable storage media (e.g. read-only memory devices within acomputer, such as ROM, or devices readable by a computer I/O attachment,such as CD-ROM or DVD disks), information alterably stored on writablestorage media (e.g. floppy disks, removable flash memory and harddrives) or information conveyed to a computer through communicationmedia, including wired or wireless computer networks. In addition, whilethe invention may be embodied in software, the functions necessary toimplement the invention may alternatively be embodied in part or inwhole using firmware and/or hardware components, such as combinatoriallogic, Application Specific Integrated Circuits (ASICs),Field-Programmable Gate Arrays (FPGAs) or other hardware or somecombination of hardware, software and/or firmware components.

While the invention is described through the above-described exemplaryembodiments, it will be understood by those of ordinary skill in the artthat modifications to, and variations of, the illustrated embodimentsmay be made without departing from the inventive concepts disclosedherein. Furthermore, disclosed aspects, or portions of these aspects,may be combined in ways not listed above. For example, the spectrometerdescribed above may be used in other contexts, such as terrestrial orextraterrestrial astronomy, including in combination or withintelescopes and satellites. Accordingly, the invention should not beviewed as limited.

1. A method for mounting an optical element, comprising: providing anoptical mount comprising a carbon-filled polymer; and attaching theoptical element to the optical mount.
 2. A method in accordance withclaim 1, wherein the carbon-filled polymer comprises graphite-filledpolyphenylene sulfide.
 3. A method in accordance with claim 1, whereinthe carbon-filled polymer comprises polyphenylene sulfide filled with atleast about 40% carbon.
 4. A method in accordance with claim 1, whereinthe optical element comprises a lens.
 5. A method in accordance withclaim 1, wherein the optical element comprises an optical sorter.
 6. Amethod in accordance with claim 1, wherein the optical element comprisesa diffraction grating.
 7. A method in accordance with claim 1, whereinthe optical element comprises a prism.
 8. A method in accordance withclaim 1, wherein the optical element comprises a mirror.
 9. A method inaccordance with claim 1, wherein the optical element comprises a mask.10. A spectrometer, comprising: a carbon-filled polymer structuralmember; a light dispersion element mounted to the structural member; anda sensor mounted to the structural member and oriented to receivedispersed light from the light dispersion element.
 11. A spectrometer inaccordance with claim 10, wherein the carbon-filled polymer comprisespolyphenylene sulfide filled with at least about 40% carbon.
 12. Aspectrometer in accordance with claim 10, further comprising: an input;and an order sorter disposed between the input and the light dispersionelement.
 13. A spectrometer in accordance with claim 12, furthercomprising a structure defining an aperture disposed between the ordersorter and the light dispersion element.